This invention relates to a multi-layer polypropylene film having a low film to film coefficient of friction.
Highly crystalline polypropylene film is an excellent packaging material, but it has a high film to film coefficient of friction which makes it difficult or impossible for it to be successfully utilized in automatic packaging equipment. The film will not respond to the packaging speed capability of the system and, as a consequence, jamming results.
In the past, the coefficient of friction characteristics of polypropylene films have been beneficially modified by the inclusion in the polymer of coefficient of friction (COF) additives such as, for example, fatty acid amides. The effectiveness of an amide depends upon its ability to migrate to the surface of the films in order to reduce COF. While such amides do improve the COF of the films, the value of the COF is subject to wide variation depending upon the heat history which the film experiences during storage, shipping and certain converting processes. The presence of such amides on the film surfaces can adversely affect the film's appearance as manifested by an increase in haze, a decrease in gloss and the presence of streaks. The presence of such amides on the surface can also adversely affect the wettability and adhesion of solvent and water base inks, coatings, adhesives and metals.
In an effort to improve the COF of polypropylene films without adversely affecting the films' appearance, wetting and adhesion, it has been proposed to prepare a polypropylene film having on at least one surface thereof a layer of a blend of (1) a member selected from the group consisting of a medium density polyethylene (MDPE), a high density polyethylene (HDPE) and mixtures thereof, and (2) polypropylene. (U.S. Pat. No. 4,578,316 to Clauson et al.) This patent discloses that the two materials in the surface layer can be blended in a percent by weight of from about 2% by weight to about 60% by weight of the medium or high density polyethylene with the remainder being the polypropylene. The patent also suggests that at percentages of 60 to 100% MDPE or HDPE, the surface layers are too soft and too hazy. That such high (MDPE or HDPE) content films would have comparatively inferior optical properties is not unexpected, because MDPE and HDPE films are known to be hazy and opaque.